1. Field of the Invention
The present invention relates to a relocation system and a relocation method capable of relocating a virtual volume that is formed based on thin provisioning while ensuring security against exhaustion of pools.
2. Description of Related Art
In general, data managed in an organization such as a company is stored in a storage device called a disk array subsystem or the like. This storage device has a plurality of storage units such as hard disk drives to form Redundant Array of Inexpensive Disks (RAID) on which logical storage areas called volumes are formed. A host computer (hereinafter referred to as “host”) accesses the storage units via these volumes to read/write data.
In recent years, amount of data managed in organizations such as companies continues to increase year after year and effective utilization of storage devices has become an issue of great importance. To address this issue, there are conventional techniques like tiered storage management and thin provisioning, and thin provisioning has become a technique for increasing efficiency of utilization of storage device capacity.
JP-A-2006-99748 discloses a method for tiered storage management that classifies volumes into groups called tiers according to their characteristics and appropriately positions data in a volume that has characteristics appropriate for the value of the data so as to effectively utilize storage.
For a general volume not based on thin provisioning (hereinafter called “general volume”), an area equivalent to the total capacity of the volume is allocated in a storage unit when creating volumes. On the other hand, in thin provisioning, a plurality of general volumes are pooled and virtualized volumes (hereinafter “virtual volume”) are formed thereon. The virtual volumes are then exposed to the host and a physical storage area is allocated from a general volume in the pools and assigned only to a data block in which data is actually written.
Its advantage is that, even when virtual volumes of a large capacity are created, only general volumes that are actually used by the host have to be prepared, which increases efficiency of utilization of storage device capacity. A drawback is that when available space in general volumes within a pool runs out, it is no longer possible to write data to all virtual volumes formed in that pool. Thus, a user (or a manager of the storage device) needs to estimate increase in usage of a pool and perform maintenance operations as necessary such as addition of general volumes to the pool so that the pool does not run out during operation.
Use of thin provisioning and tiered storage management in combination would promote effective utilization of storage. That is, it is expected that, by classifying pools into tiers according to their characteristics and relocating virtual volumes among a plurality of pools that belong to different tiers, users can enjoy advantages of both thin provisioning and tiered storage management.
The conventional technique mentioned above has problems as described below and is far from being able to adequately support effective utilization of storage by users when applied to thin provisioning.
A first problem is that risk of pool exhaustion increases. This is because it is difficult to estimate increase in usage amount of pools after relocation since trend in usage amount of the individual pools largely changes as a result of relocating a virtual volume among the pools. In the worst case, it is possible that, as a result of executing relocation, a time period before a destination pool runs out (hereinafter referred to as “remaining operation period”) becomes short and a user cannot take action in time.
A second problem is that relocation is difficult to utilize as means for adjusting available space in pools. For example, if the user finds a pool that is likely to run out soon and tries to extend the time before its exhaustion by moving a virtual volume formed in that pool to another pool, it is difficult to determine which virtual volume should be relocated to which pool in order to extend the time before its exhaustion most effectively.